BEIRUT — Syrian rebels launched an offensive Sunday in an eastern city near the border with Iraq in an attempt to extend their advances in the north and west of the country, activists said.

The Britain-based Syrian Observatory for Human Rights and the Local Coordination Committees said that Syrian army warplanes also conducted several air raids against rebel positions in the eastern city of Deir el-Zour.

The LCC and the Observatory said rebels killed seven soldiers and captured several others in the city that has been contested since last year.

Rebels have been on the offensive in northern Syria where they captured the town of Khan al-Assal last month. Last week, opposition fighters captured 11 villages in the regime stronghold of Latakia province along the Mediterranean coast, a symbolic blow to President Bashar Assad.

“Fighters are trying to capture neighborhoods in Deir el-Zour but so far they have not been able to,” said Rami Abdul-Rahman, who heads the Observatory. “They are trying to take the whole city.”

So far, rebels have only been able to fully capture one provincial capital, the northern city of Raqqa. They hold parts of several other major cities, including the northern city of Aleppo, Syria”s largest, as well as Deir el-Zour and the central city of Homs.

Syria”s state-run news agency SANA said government forces inflicted losses among “terrorists” in Deir el-Zour including foreign fighters. The Syrian government denies there is an uprising in the country and says Syria is being subjected to a foreign conspiracy.

SANA reported later Sunday that rebels shelled the central town of Salamiyeh, killing at least 11 people and wounding 20 others. Salamiyeh, where most residents belong to the Ismaili branch of Shiite Islam, is under regime control.

Syria”s conflict has taken on an increasingly sectarian tone in the last year, pitting predominantly Sunni Muslim rebels against members of Assad”s Alawite sect, an offshoot of Shiite Islam.

The Observatory reported Sunday that a rebel group captured 13 Syrian Kurds near the town of Tel Aran in the northern province of Aleppo and handed them over to members of al-Qaida-linked Jabhat al-Nusra or Nusra Front.

It said the detained Kurds were tortured.

Aleppo and the northeastern province of Hassakeh have witnessed heavy fighting in the past months between members of al-Qaida-linked jihadi groups and Kurdish gunmen that left scores of people dead on both sides.

On Saturday, the president of Iraqi Kurdistan, Massoud Barzani, vowed to defend the large Kurdish population in Syria from al-Qaida-linked rebel fighters, highlighting the potential for Syria”s civil war to morph into a full-blown regional, ethnic and sectarian conflict.

The fighting in the oil-rich region near the Iraqi border has emerged as yet another layer in Syria”s increasingly complex and bloody civil war.

Unrest in Syria began in March 2011 and later exploded into a civil war. More than 100,000 people have been killed in the conflict.